Bit streams conveying sequences of binary words are used to represent a variety of message forms. These include sensor outputs, text, audio signals, video signals. In many applications, wireless communications is the preferred form of transmitting these bit streams from a source to a user. Wireless communications uses sinusoids as propagating waves. These sinusoids are modulated by the bits in the stream. In this way, the bits are carried by the propagating wave. A common form of modulation involves variation of the frequency of the sinusoid to create frequency modulation (FM), sometimes referred to as frequency shift keying (FSK) when the bits in the stream are distinguished by two or more voltage levels.
In all forms of carrier modulation, an increase in the rate at which the bits are sent requires an increase in the bandwidth of the modulated sinusoidal carrier. The proliferation of the use of bit streams to convey messages creates ever increasing demands for additional spectrum (bandwidth).
It has been demonstrated that it is possible to recover messages from each of two or more co-channel FM carriers. This provides a spectrum reuse capability. Circuits previously developed as demodulators of each of two co-channel FM carriers can be used when the message is analog or when the message is a digital bit stream. Consequently, these circuits do not utilize prior information available in the bit stream format. Further, these general purpose circuits require adjustment when parameters of the communication system vary. Therefore, there is a need to develop receiving systems which are simple and robust for the important case of independent bit streams as messages on each of two co-channel carriers.
U.S. Pat. Nos. 4,859,958 and 4,992,747, invented by Glen A. Myers, the inventor of the present invention, are each incorporated by reference in the present application as though fully set forth herein. In these patents, a means for demodulating all of several FM carriers is described.
U.S. Pat. No. 5,038,115, co-invented by the inventor of the present invention, is also incorporated by reference in the present application as though fully set forth herein. In this patent, phase tracking of input terminal signals is described. In one embodiment of the phase tracking circuit of U.S. Pat. No. 5,038,115, a phase tracking circuit makes use of two phase-locked loops electrically connected in a feed forward manner.
U.S. Pat. No. 5,329,242 invented by the inventor of the present invention, is also incorporated by reference as though fully set forth herein. In this patent, demodulating a frequency modulated signal using the time intervals between zero crossings of a received carrier signal is described. Averaging and mapping techniques are used to improve estimates of the message signal.
U.S. patent application 08/214,378 by the inventor of the present invention is also incorporated by reference as though fully set forth herein. This application describes, analytically and geometrically, the effect of adding two sinusoids of different frequency.
U.S. patent application 5,541,959 by the inventor of the present invention is also incorporated by reference as though fully set forth herein. In this patent are described a method and apparatus for creating a replica of a dominant carrier.
U.S. patent application 5,554,955 by the inventor of the present invention is also incorporated by reference as though fully set forth herein. This patent describes method and apparatus for removing the effects of co-channel interference from the message on a dominant frequency modulated carrier and for recovering the message from each of two co-channel carriers. The messages can have analog or digital formats.
There is a need for a technique which can use only the output of any type of frequency demodulator to recover independent bit streams from each of two frequency modulated co-channel carriers.